which is more musical? A REL or a SUMIKO subwoofer for two channel only?

Both. Although two of each will be much more musical. Or three of one and one of the other. Actually, anything where you wind up with at least four is more musical than anything with just one.

Specify "why"? Okay. Because when it comes to subs no matter which one you get they all have the same problem of room modes that always make the bass lumpy. Way too much one place, way too little the other. Even with EQ, impossible to ever get it right.

Multiple subs however go in multiple locations and produce a lot of different modes in a lot of different locations, the overall effect of which is much smoother, faster, more articulate bass. Its not even close.

So get whichever is cheaper, because then you can afford more, and more is what you want.

AZ Jake when we attempt subwoofers the best ways for 2 channels of music
is the way Rel
 or Sumiko’s hook up at high level. The only thing they both forgot is the High Pass filter . In order to achieve an even better Transition the use of a sophisticated  high pass filter like the Vandersteen Sub 3 and prior model 2WQs use this in particular allows the main speakers modulation distorting to be lowered and heavy lifting of your main amplifier to be relieved dramatically lowering distortion Improving Clarity and transparency of the whole system.
The sub three also features 11  analog EQ pots cleans up the in room bass response and done without any DPS processor The same analog system is also used in the more expensive Vandersteen Quatros five series and Sevens and has now trickled down.
best JohnnyR 
Vandersteen Dealer 

I've had both brands for high-end two-channel systems.
I ***STRONGLY*** suggest you go with REL!
Believe me, in the long run, you will be much happier.